Controlled Molar Inclination during Maxillary Dental Expansion Using a Straight Rectangular Archwire

Introduction

In adult patients, in whom growth has ceased, maxillary dental expansion is commonly used to correct mild or moderate transverse maxillary deficiencies.^[1,2] Appliances such as the quad helix, nickel-titanium (Ni-Ti) palatal expander, overexpanded archwire, and removable plates with expansion screws are typical treatment modalities.^[3,4] However, a common side effect of dental expansion is uncontrolled buccal crown tipping, which generates high stress on the buccal alveolar bone crest and palatal root apex of the posterior maxillary teeth.^[1] Periodontal problems, such as buccal dehiscence on the maxillary posterior teeth, have also been reported after expansion.^[4,5] Moreover, palatal cusp extrusion may cause increased posterior vertical dimension of occlusion, bite opening in the anterior region, mandibular posterior rotation, and worsening of the facial profile.^[4] According to a study that assessed the Ni-Ti palatal expander, approximately 8° of maxillary first molar tipping and 1.4° increase in SN-MP occurred in participants.^[6]

The use of a rectangular overexpanded stainless steel archwire has been suggested as a way to prevent buccal crown tipping.^[3] When inserted into the brackets, the rectangular shape of the wire produces a moment around the teeth. This counteracts the tipping effect induced by the expansion force, leading to reduced buccal tipping of the posterior teeth.^[3] However, stainless steel wire is characterized by a high load deflection rate and stiffness, which may cause a large increase in force, even if a small amount of deactivation occurs.^[7] Attempts to use a lighter magnitude of force have been made. A recent case report demonstrated the successful application of a straight 0.032” round beta-titanium alloy wire overlaid on a 0.016” × 0.022” Ni-Ti archwire to simultaneously align and expand the dental arch.^[8] However, the magnitude of the expansion force was not quantified.

We chose to use rectangular beta-titanium alloy (TMA^®) wire as an alternative to stainless steel wire, as TMA^® wire has a lower load deflection rate and stiffness.^[7] Based on pilot laboratory tests on ten pretreatment maxillary models requiring dental maxillary expansion (intermolar width of 43–47 mm), straight 0.016” × 0.022” TMA^® wires were bent into an arch. These were found to produce 126 ± 27 cN of expansion force and successfully promoted dental expansion in our pilot clinical test. From these results, we postulated that, with the torque control effect of the rectangular wire, molar inclination should be controlled regardless of the amount of expansion.

The main objective of this study was to examine whether the amount of expansion has an effect on the change in molar inclination when a straight rectangular wire is used to expand the maxillary dental arch. Our null hypothesis was that the straight rectangular wire would have no effect on the ability to control molar inclination during maxillary dental expansion.

Materials and Methods

Study protocol

At the leveling and aligning stage, the maxillary teeth were bonded with 0.018” ×0.025” slot preadjusted edge-wise brackets from the left to right second premolars (Roth system, Master Series™, American Orthodontics^®, Sheboygan, WI, USA), and molar buccal tubes (nonconvertible, LP™, American Orthodontics^®, Sheboygan, WI, USA) were bonded to the left and right maxillary first molars. After bonding, series of 0.012” Ni-Ti, 0.014” Ni-Ti, and 0.016” × 0.016” Ni-Ti were delivered to align the maxillary teeth. When the maxillary teeth were leveled and aligned (T0), an alginate impression (Alginoplast^®, Heraeus, Hanau, Germany) of the maxillary teeth was obtained following the manufacturer’s instructions, and then poured within 1 h with dental stone (Atlas^®, ULTIMA, Seiches-sur-le-Loir, France) to make the maxillary models. The maxillary dental arch was expanded using a straight 0.016” × 0.022” beta-titanium alloy wire (TMA^®, Ormco™, Orange, CA, USA) [Figure 1]. The TMA^® wire was attached to all brackets using ligature wires. At subsequent visits, which were scheduled every 3–4 weeks, the wire was removed, straightened, re-engaged in the brackets, and ligature ties were replaced. At each visit, the periodontal health of all maxillary teeth was assessed.

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Figure 1

Dentoalveolar maxillary expansion with a straight rectangular TMA® wire. (a) The straight TMA® wire is inserted into brackets and buccal tubes on the right to left maxillary first molar. (b) Wire ligatures are used for attachment to all brackets

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When the predetermined maxillary posterior arch width had been achieved (T1), alginate impressions were made to fabricate the maxillary models.

Model analysis

All reference models were digitized using an orthodontic three-dimensional scanner (R700 model, 3Shape; Ivoclar Vivadent, Inc., Copenhagen, Denmark). The second maxillary molars were cut off with Ortho Analyzer software (3Shape, Ivoclar Vivadent, Inc., Copenhagen, Denmark) [Figure 2]. The prepared digital models were used to measure the amount of expansion and molar inclination.

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Figure 2

Three-dimensional digital maxillary model preparation using Ortho Analyzer software. (a) The original scanned model. (b) The digitized model with the second maxillary molars cut off

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The amount of expansion was measured directly on the digital maxillary models. The following distances were measured: intercanine width (U3–U3) between the cusp tips of the right and left maxillary canines; first and second interpremolar widths (U4–U4 and U5–U5) between the central grooves of the right and left first and second premolars, respectively; and intermolar width (U6–U6) between the central fossae of the right and left maxillary first molars [Figure 3a]. The expansion rate was calculated as the amount of expansion (T1 – T0) divided by the expansion time (months).

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Figure 3

(a) Reference points used to determine the amount of expansion, intercanine width (U3–U3), first and second interpremolar widths (U4–U4 and U5–U5), and intermolar width (U6–U6). (b) Maxillary first molar inclination measurement

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The molar inclination was the angle between two lines drawn from the mesiobuccal cusp through the mesiopalatal cusp of the right and left maxillary first molars [Figure 3b].

All data were measured by the same investigator who was blinded to the participants’ identity and the sequence of materials being measured. All data for ten randomly selected participants were measured at two time points (4 weeks apart) to assess accuracy and reliability.

Results

Nine males and 11 females with a mean age ± SD of 19.5 ± 2.1 years were recruited into the study. There was no significant difference in arch width between males and females [Table 1]; therefore, the data for male and female participants were pooled.

All arch width parameters significantly increased between T0 and T1 (P < 0.05). In descending order, the amount of expansion and the rate of expansion were highest at U6–U6 (2.4 ± 0.8 mm and 0.8 ± 0.3 mm/month, respectively), U5–U5 (2.3 ± 1.0 mm and 0.7 ± 0.3 mm/month, respectively), U4–U4 (1.7 ± 0.8 mm and 0.5 ± 0.3 mm/month, respectively), followed by U3–U3 (0.8 ± 0.5 mm and 0.2 ± 0.1 mm/month, respectively) [Table 2].

There were no significant differences in age or initial molar inclination between participants in the minor and moderate expansion groups [P > 0.05; Table 3], although there was a statistical difference between groups for initial arch width (P < 0.05). For the change in molar inclination, there was no significant difference between groups (P > 0.05). The molar inclination change of all participants was 2.1° ± 1.2°.

Discussion

This study showed that the use of a straight rectangular beta-titanium alloy wire, producing an estimated applied force of 126 ± 27 cN, can induce transverse expansion of the posterior teeth in adult patients while controlling molar inclination.

The use of a straight rectangular beta-titanium alloy wire in this study resulted in decreased buccal tipping of the first molars (overall change of 2.1° ± 1.2°) in comparison with other studies that employed Ni-Ti palatal expanders (3°–5°)^[6] or quad helix appliances (6.1° ± 4.7°).^[10] These differences may be due to the greater expansion required in other studies, variation in measurement methods, or the improved ability of our system to control molar inclination during expansion. We used a 0.016” × 0.022” wire in 0.018” slot brackets which allowed only 0.002” of play between the wire and bracket slot, thereby limiting the amount of buccal crown tipping of the first molars. Another possible reason for the reduced first molar buccal crown tipping observed in our study is that the moment of the expansion force from the straight rectangular wire may have been greater than the moment of the couple producing the palatal crown torque moment.

A previous study showed that molar tipping increases with the extent of molar expansion,^[6,10] which suggests that the counter moment provided by other expanders is not sufficient to prevent tipping of the molars. The expansion method used in the current study appears to have created an adequate counter moment to limit molar tipping, so it did not exceed the clinically important range of intermolar width expansion of up to 4 mm.

This study used cusp tips as the reference points for measuring changes in molar inclination. Measurements performed using these landmarks can be affected by occlusal attrition. A prospective comparative study with other types of expanders and a larger sample size is required to confirm the efficacy, benefit, and cost-effectiveness of the straight wire technique employed in this study. In this study, the maxillary expansion technique using a straight 0.016” × 0.022” beta-titanium alloy wire successfully increased the maxillary arch width. As this technique increased the intermolar and intersecond premolar widths to a greater extent than the intercanine width, this method may be appropriate for the treatment of individuals with constriction of the maxilla in the intermolar area.

A limitation of this technique is that it may not be appropriate for patients with severe crowding, a posterior crossbite, or a tendency toward an anterior open bite. Another clinically important factor is that, although this technique appears to be effective, the appliance must be frequently monitored and not be left engaged for too long as there are no self-limiting or other safety features to prevent continued active forces and potential iatrogenic amounts of expansion if, for example, the patient misses appointments or withdraws.

Conclusions

The use of a 0.016” × 0.022” straight beta-titanium alloy wire engaged in a fixed orthodontic appliance was able to effectively expand the maxillary dental arch in this group of patients. The straight rectangular wire controlled molar inclination within the range of 1.0–4.0 mm.